The study appears to demonstrate that neither clipping or undercutting affects the final plants at maturity, when compared to transplants that were not clipped and not undercut. I've noticed during my own transplants from 1020 tray inserts that when roots extending out the bottom of a cell are inadvertently amputated in the process of removing the plug from the insert cell, it seems to make no difference, compared to those transplants whose extending roots were lifted out intact with the soil plug. I also have noticed no difference in the duration of transplant shock—usually about 1-2 weeks.
Bob